Single phase flow line sizing

Single-phase fluid flowGUIDELINE TO PIPE SIZING FOR SINGLE-PHASE FLOW

AUTHOR: VIKRAM SHARMADATE: 7th MARCH 2017

Table of ContentsIntroductionBernoulli’s EquationFriction factor graphPressure Drop in Pipe and FittingsRecommended fluid velocitiesCalculation MethodologyReferences

IntroductionPipe is a common sight in chemical plantPipework & fittings:

◦ ≈ 20-30% of the total design cost; ◦ ≈ 10-20% of the total plant investment; and◦ Other added cost due to maintenance req. & energy

usage in the form of ΔP in the fluids being pumpedThe size of a pipe (diameter) is expressed in

two ways that are:◦ Nominal Pipe Size (NPS); and◦ Diametre Nominal (DN)

NPS is measured in inches, DN is measured in mm

DN is generally equiv. to NPS multiplied by 25

Introduction (cont’d)DN is generally equiv. to NPS multiplied by 25

except:◦ NPS ½ is DN15◦ NPS 3 is DN80

Pipe size are designated by two numbers that are (i) pipe diameter and (ii) thickness

Pipe diameter is generally associated with inside dia.

Outside dia. is the same for a given size → maintain certain interchangeability of pipe fittings

NPS 14 and beyond, it is equal to the outside dia. (OD) in inch.

Introduction (cont’d)NPS 14 and beyond, it is equal to the outside

dia. (OD) in inch. (cont’d)Pipe wall thickness is referred to pipe

schedule (Sch)Standardize from 5 to 160 → determined by

the service req. like Pressure, Temp., Flow and corrosion

Pipe wall thickness ↑ Pipe Schedule ↑

Bernoulli’s Equation ΔP or head loss in a piping system is caused by:

◦ Elevation;◦ Friction;◦ Shaft work; and◦ Turbulence due to sudden change in direction or cross

sectional area Mechanical Energy Balance (MEB) eq.→ conservation of sum

of pressure, kinetic and potential energies, net heat transfer (q), work done by the system (w) and frictional energy (ef).

ef is usually +ve & represents the rate of irreversible conversion of mech. energy into thermal energy

Sometimes called head loss, friction loss or frictional pressure drop.

Bernoulli’s Equation (cont’d)

Bernoulli’s Equation (cont’d) The first 3 terms (pressure, velocity & elevation) are

point functions → depend only on conditions at the inlet & outlet of the system

w and ef are path functions → depend on what is happening to the system between the inlet and outlet points

ef loss due to friction and includes losses due to flow through lengths of pipe, fittings such as elbows, valves, orifices and pipe entrances and exits

Bernoulli’s Equation (cont’d)

Kf is the excess head loss due to pipe or pipe fittings, v is the fluid velocity

Fluids flowing through pipes;

ΔP is the same due to flow is the same whether the pipe is horizontal, vertical or inclined

Friction factor

Friction factor is expressed as Moody friction factor (fM) or Fanning friction factor (fF).

fM = 4fF

Friction factor (cont’d) Laminar region (or viscous): Re < 2,000 “Critical zone” is the transition frm. laminar to

turbulent: 2,000 < Re < 4,000. Turbulent flow: Re > 4,000 Friction factor is laminar region is not affected by the

relative roughness (ε/D) but it is influenced by the fluid viscosity

Friction factor at transition region strongly depends on both the Re and ε/D

Friction factor at turbulent region is independent of Re and is a function of relative roughness (ε/D).

Friction factor (cont’d) Friction factor can be computed using two equations

based on the fluid flow regime. Laminar flow (Re < 2,000) : Turbulent flow (Re > 4,000): Reynolds Number:

◦ fD = Darcy friction factor◦ ε = Absolute pipe roughness (m):

Carbon Steel = 0.02-0.05 mm Stainless steel = 0.03 mm

◦ D = Pipe inner diameter (m)◦ ρ = Fluid density (kg/m3)◦ µ = Dynamic viscosity (Ns/m2)

Friction factor (cont’d) Colebrook’s equation require iteration to determine the

friction factor. Author propose Churchill (1977) equation that allows

engineers to determine the friction factor for both laminar and turbulent flows.

Pressure Drop in Pipe and Fittings ΔP for straight pipe run: Pressure drop for fittings can be computed using

resistance coefficient method (K). This could be done via Darby 3-K method for fittings

Why Darby 3-K method is preferred?◦ Accounts directly for the effect of both Re & fitting size on the

loss coefficient.

Pressure Drop in Pipe and Fittings (cont’d) Other option: The Equivalent Length Method (Leq/D) The Leq/D method assumes that:

◦ Size of fittings of a given type can be scaled corresponding to a given dia.◦ Reynolds number on the friction loss is the same as the pipe loss

The above assumptions are incorrect. Why? The laminar or turbulent flow within a valve or a fitting

is generally quite different from that of straight pipe. With this, there is an uncertainty when determining the

effect of Re on the loss coefficients. Leq/D method does not account for the lack of exact

scaling for valves and fittings

Pressure Drop in Pipe and Fittings (cont’d)For Darby 3-K constants, refer to:

◦ https://neutrium.net/fluid_flow/pressure-loss-from-fittings-3k-method/

For other fittings such as sudden pipe contractions, square reduction, tapered reduction, sharp orifice, square expansion, tapered expansion, thick orifice and pipe reduce, refer to Coker (2007)

Recommended fluid velocitiesTypical velocities and pressure drop:

◦ Liquid (pumped, not viscous): 1-3 m/s, 0.5 kPa/m◦ Liquid (gravity flow): - m/s , 0.05 kPa/m◦ Gases & vapours: 15-30 m/s, 0.02 % of the line

pressure◦ HP steam (> 8 bar): 30-60 m/s, - kPa/m

Typical velocities at pump suction and discharge lines:

Recommended fluid velocities (cont’d)Typical velocities and pressure drop for

single-phase gas process lines:

Calculation Methodology Obtain the piping layout from PFD, P&IDs or Piping

Isometrics Obtain data at fluid inlet of the pipe corresponding to

inlet temperature and pressure. Info req. are mass flow, ρ, µ, T & P.

Select a pipe size and material (Refer to Slide #5) Calc. the fluid velocity. Ensure it comply with the fluid

req. (Refer to Slide #17) Calc. Re → Laminar or Turbulent (Refer to Slide #12) Calc. the fD → Churchill’s (1977) eq. (Refer to Slide #13) Calc. the ΔPbar/100m of the straight pipe. Include elevation

(Refer to Slide #14). Calc. ΔPbar by multiplying with pipe straight length

Calculation Methodology (cont’d)Calc. ΔP of fittings. Use Darby 3-K method and

Coker (2007) (Refer to Slide #16 and Slide #14)

Calc. ΔP of other items such as process equipment etc.

Calc. the total pressure drop of the system (ΣP)◦ ΣPT = ΔPfriction + ΔPfittings + ΔPother items

Check if downstream pressure P2 is as per specifications. ◦ P2 = P1 - ΣPT

If not, repeat the above calc. by selecting a different pipe size

References Bahadori, A. (2014). Process pipe sizing for plants

location. In Natural Gas Processing: Technology and Engineering Design (p. 83). Oxford: Gulf Professional Publishing.

Murty, K. K. (2010). Sizes, Schedules, And Standards. In All-in-One Manual of Industrial Piping Practice and Maintenance On-The-Job Solutions, Tips and Insights (p. 52). New York: Industrial Press.

Native Dynamics. (2012, May 19). Neutrium. Retrieved March 7, 2017, from Absolute Roughness of Pipe Material: https://neutrium.net/fluid_flow/absolute-roughness/

Sinnot, R. K. (2005). Piping and Instrumentation. In Chemical Engineering Design (Vol. 6, p. 218). Oxford: Elsevier.